I have organised my (many!) figures in a file listing them all
(\useexternalfigure[][][width=...])
I find it very convenient to adjust the width there and not in the
\placefigure part of the single chapters. I suppose it is presently
not possible to also control the placement (inner, outer and such)
from \useexternalfigure. Of course I have no idea whether that would
be possible/difficult/easy. But if it's easy to implement, just take
it as a (non-urgent) suggestion.
Cheers, Jörg